This proposal aims to define the immunochemical parameters underlying clinical ABO hemolytic disease of the newborn and devise a suitable prenatal test with which to anticipate its occurrence. Both prospective and retrospective approaches will attempt to relate molar concentrations of maternal IgG anti-A and Anti-B, number of specific antigenic sites on red cells of involved babies and their fathers, and IgG subtype (IgG1 and IgG3 versus IgG2 and IgG4) of maternal antibodies bound by fetal red cells.